In a time division multiplexing system sampled or encoded speech signals originated by a subscriber occupy assigned time slots in a repetitive, ordered group of time slots termed a "frame". Connection of two subscribers is achieved by a number of techniques the end result of which is that the information contained in the respective time slots of the two subscribers are bilaterally exchanged. Should three or more subscribers desire simultaneous multilateral connection, i.e. a conference connection, more complicated processing of the time slot information is necessary.
Given a PCM system, the obvious way to realize a conference between subscribers is to algebraically sum the PCM words of the participants except one, and to transmit the sum to the one participant excepted. If this procedure is performed once every frame for each conference participant, a conference connection would have been established. Should the PCM signals be a result of nonlinear encoding (which is usually the case), it would be necessary to linearize the PCM words before they can be summed and to convert each resultant sum to the nonlinear code again prior to its transmission to the appropriate conference participant.
It is because of the foregoing complications and the required expensive circuitry that the prior art, for example U.S. Pat. No. 3,699,264 issued Oct. 17, 1972 to Pitroda et al. shies away from such technique. In the Pitroda patent a simpler approach is chosen; namely, the participant with the numerically largest binary PCM word is chosen as speaker by a multilateral comparator circuit. While this simple approach may be less costly, it is often inadequate in that it does not permit natural multilateral conferencing.